1. Field of the Invention
The present invention generally relates to light amplifiers, and more particularly to a light amplifier suitable for a situation in which a light amplifying operation is performed in a transmission path.
In optical communications in which signals are wavelength-multiplexed, it is required that the gain of a light amplifier is constant with respect to the wavelengths of signal lights.
2. Description of the Related Art
A light output control has been reduced to practical use in which a light amplifier regulates the light output to a constant level. However, the dependence of the gain of the light amplifier with respect to the signal wavelengths is still in the researching stage.
Several control methods for controlling the wavelength-dependence of the light amplifier gain have been proposed. For example, the composition of an amplifying medium is varied, or amplifying media having different compositions are combined together. Another proposed method is to use a filter having the function of compensating for the wavelength-dependence of the amplifying medium.
However, the known control methods cannot remove the dependence of the wavelength-dependence of the light amplifier with respect to the intensity of an incident light. In this regard, a light amplifier utilizing a rare-earth-doped glass has been proposed. Another light amplifier has also been proposed and marketed only for use in research, in which fluoride glass is used as host glass in order to reduce the wavelength-dependence of the cross section for emission and absorption. However, fluoride glass does not have a good reliability in the water resistance of fluoride glass, and has a difficulty in splicing for connections because the melting point thereof is quite different from that of normal glass. Further, there is a problem in which fluoride glass itself has a good reliability. For the above reasons, use of fluoride glass is not suitable for applications to main routes of commercial communications services.
The wavelength-dependence of the light amplifier gain has an input-light-intensity-dependence as shown in FIG. 1, which shows characteristics of an erbium-doped fiber amplifier. More particularly, FIG. 1 shows gain differences with respect to the maximum gain when sweeping the probe light between 1550 nm and 1554 nm. In the prior art, there is no consideration of the input-light-intensity-dependence of the light amplifier gain.
Normally, in the see-bottom optical filters, optical repeaters are intermittently provided in the optical fiber cable at given intervals. It is possible to equally arrange each of the optical repeaters so as to have a light amplifying characteristic compensating for the input-light-intensity-dependence, so that the wavelength-dependence of the light amplifier gain can be optimized. On the other hand, in the ground light communications systems, light repeaters are not provided at given intervals. Hence, in order to optimize the light repeaters, it is required that each of the light repeaters be optimized taking into a respective installed location. This is troublesome and reduces the advantages of use of the light amplifiers.